P20F4
Reductant Consumption Too Low
The DEF consumption observed over the dosing window is below the model expectation — typically a clogged dosing nozzle, weak pump, or leak between pump and injector that recirculates DEF back to the tank.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Common causes
Ordered by how often each turns out to be the cause.
- Dosing nozzle clogged with crystallised DEFCommon
- Pump weak — flow under targetPossible
Parts involved
- Reductant Metering Unit
- Reductant Pump
- Reductant Injection Valve
P20F4 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.